The manufacture of Portland cement results in the generation of extremely large tonnages of waste dust known as cement kiln dust. Excessive pollution when this dust is allowed to go up the stack and into the atmosphere, as well as the economic considerations involved in its loss, has required that this dust be trapped and collected by electrostatic precipitators or other means. Efforts to re-use the dust by returning it to the kiln in the form in which it is collected have not been entirely satisfactory for two basic reasons.
Firstly, such is in such fine form that a high proportion of it is again carried up the exhaust gases before it enters into the cement-forming reaction and thus lost to the operation, as well as placing an excessive burden on the stack dust-collecting system.
But more importantly, the waste dust contains an excessively high alkali metal sulphates and chlorides which cannot be tolerated in the making of cement of acceptable quality. As pollution standards are raised and/or stack collecting equipment is increased in effectiveness so that dust of greater fineness particle size is collected, this latter problem is intensified, as it is known from analyses that generally the alkali content increases with any increase in fineness of the dust. Procedures heretofore proposed for changing the physical form and/or lowering the alkali content of cement kiln dust have been unattractive or impractical for economic or other reasons.
For example, over 50 years ago U.S. Pat. Nos. 1,354,642 and 1,402,173 proposed to treat cement kiln dust with a calcium chloride solution to convert the alkali metal sulphates to alkali metal chlorides, separating the solution from the solids, and recovering the potassium chloride from the solution. However, the proposed process has never been well received, presumably because, inter alia, it is uneconomical, presenting the difficulty of separating the solids from the waste liquor, as well as the environmental problem of disposing of the large volume of waste liquor.